There has been a great deal of work on the refinement of microstructures based on cold-working and recrystallizing heat treatments (annealing). Unfortunately, these techniques have experienced limited success for refining pure aluminum microstructures. The highly-mobile grain boundaries in high-purity aluminum can allow spontaneous partial recrystallization to occur at room temperature under normal, ambient working conditions. In addition, high-purity aluminum does not have any precipitates or any significant amount of solute to provide the “Zener drag” necessary for effective retardation of grain boundary motion. Consequently, grain size is very difficult to control using conventional thermomechanical processing methods.
Historically, pure aluminum sputter targets have been manufactured with recrystallized grain sizes ranging typically from 500 μm to 5 mm. These “large” grain sizes can contribute to poor sputter uniformity. In addition, since these pure aluminum sputter targets have limited strength, they often require backing plates to control warping during sputtering. In view of these problems, there is a desire to improve the strength and sputtering performance for high-purity aluminum targets.
Target manufacturers have relied upon equal channel angular extrusion (ECAE) to produce fine grain microstructures. Nakashima et al., “Influence of Channel Angle on the Development of Ultrafine Grains in Equal-Channel Angular Pressing,” Acta. Mater., Vol. 46, (1998), pp. 1589-1599 and R. Z. Valiev et al., “Structure and Mechanical Behavior of Ultrafine-Grained Metals and Alloys Subjected to Intense Plastic Deformation,” Phys. Metal. Metallog., Vol. 85, (1998), pp. 367-377 provide examples of using ECAE to reduce grain size. ECAE introduces an enormous strain into a metal without imparting significant changes in workpiece shape. Although this process is effective for reducing grain size, it does not appear to align grains in a manner that facilitates uniform sputtering or provide an acceptable yield-the low yield originates from the ECAE process operating only with rectangular shaped plate and thus, requiring an inefficient step of cutting circular targets from the rectangular plate.
Another mechanical method for producing fine grain structures in metals is “accumulative roll bonding” where aluminum sheets are repeatedly stacked and rolled to impart sufficient strain required for ultra-fine grain sizes. N. Tsuji et al., “Ultra-Fine Grained Bulk Steel Produced by Accumulative Roll Bonding (ARB) Process,” Scripta. Mater., Vol. 40, (1999), pp. 795-800. The repeated stacking and rolling allows rolling to continue after the aluminum reaches a critical thickness. Although this process is useful for producing some products, it is not necessarily applicable for sputtering targets because of material purity requirements.
Researchers have explored using cryogenic working to increase the forming limits of aluminum alloy sheet panels. For example, Selines et al. disclose a cryogenic process for deforming aluminum sheet in U.S. Pat. No. 4,159,217. This cryogenic process increases elongation and formability at −196° C. In addition, similar work has focussed on increasing the formability of sheet panels for automotive applications. Key references include: i) H. Asao et al., “Investigation of Cryogenic Working. I. Deformation Behaviour and Mechanism of Face-Centered Cubic Metals and Alloys at Cryogenic Temperature,” J. Jpn. Soc. Technol. Plast., Vol. 26, (1985), pp. 1181-1187; and ii) H. Asao et al., “Investigation of Cryogenic Working. II. Effect of Temperature Exchange on Deformation Behavior of Face-Centered Cubic Metals and Alloys,” J. Jpn. Soc. Technol. Plast., Vol. 29, (1988), pp. 1105-1111.
Lo, et al., in U.S. Pat. No. 5,766,380, entitled “Method for Fabricating Randomly Oriented Aluminum Alloy Sputtering Targets with Fine Grains and Fine Precipitates” disclose a cryogenic method for fabricating aluminum alloy sputter targets. This method uses cryogenic processing with a final annealing step to recrystallize the grains and control grain structure. Similarly, Y. Liu, in U.S. Pat. No. 5,993,621 uses cryogenic working and annealing to manipulate and enhance crystallographic texture of titanium sputter targets.